As a result of rapid developments in electronic devices in recent years, the electronic devices have become more sophisticated in function, able to work with high performance and yet downsized. In accordance with this, electronic parts are required to be downsized and lighter in weight. For this reason, in terms of wiring boards on which the electronic parts are mounted, flexible printed wiring boards (hereinafter, abbreviated as FPCs) that have flexibility, draw attention than ever before, compared with generally-used rigid printed wiring boards. Thus, demand for FPCs rapidly is increasing recently.
Incidentally, most of FPCs are produced from a copper-clad laminate (CCL). On a surface (conduction surface) of such an FPC having a CCL, a circuit (pattern circuit) is uncovered, which is made of copper and patterned in a predetermined pattern. There is a possibility that the pattern circuit thus uncovered is deteriorated. Thus, a cover lay film (insulating protective film) is layered on the surface. The cover lay film also has a function to attain better flexibility.
As a method of layering the cover lay film on the surface of the FPC, the following method is generally used: first, the cover lay film is processed into a predetermined shape, and an adhesive layer is formed on one surface thereof; the cover lay film is placed on the FPC with accurate positioning; then, the cover lay film is thermo compression-bonded on the FPC by pressing or the like.
However, in the generally-used method, an epoxy adhesive agent, an acrylic adhesive agent, or the like is used popularly. Some of those adhesive agents have low heat resistance property such as soldering heat resistance, adhesive strength when subjected to a high temperature, whereas the others are poor in flexibility. Thus, there is a problem that use of those adhesive agents prevents full utilization of property of a polyimide film used as the cover lay film.
Moreover, in case where the cover lay film is processed into a predetermined shape before layering, it is necessary that the cover lay film have a hole or a window in such a position that corresponds to a junction section of the pattern circuit on the surface of the FPC, the junction section being a section to which a terminal section or various parts are to be jointed. However, it is difficult to form a hole or the like in the cover lay film that is so thin. Further, the hole or the like of the cover lay film is almost manually positioned with the jointing part for the terminal section. Thus, it is a problem that such manual operation reduces not only workability in layering the cover lay film on the FPC, but also accuracy in positioning, and further such manual operation leads to a high cost.
Furthermore, as a method other than the method in which the cover lay film is processed before layering, known is a method in which the cover lay film is processed by etching after layering. To explain specifically, a hole, a window, or the like is formed in a predetermined position of the cover lay film by laser etching, plasma etching, or the like method, after the cover lay film is adhered to the FPC by thermo compression bonding. This method improves accuracy in positioning. However, it is a drawback of this method that the formation of the hole or the window in this method takes a long time and requires an expensive processing apparatus and a higher production cost.
Use of a photosensitive resin composition is known as an art to solve the problems. Specifically, the photosensitive resin composition is used as follows:
(1) photosensitive resin composition is applied on a surface of an FPC so as to form a resin layer thereon and to use the resin layer as the cover lay layer; or
(2) a photosensitive film produced in advance is layered on a surface of an FPC, then exposed to light and developed with a photomask used, so as to form a resin layer thereon and use the resin layer as a cover lay layer.
Moreover, besides the formation of the cover lay layer, another usage of the photosensitive film is to use the photosensitive film as a film-like shaped photo resist for use in forming, by etching copper foil, a circuit made of copper. In this case, the photosensitive film is peeled off from a printed wiring board or the like after used as a photo resist for the etching.
In the use of such photosensitive resin composition, it is possible to utilize the photosensitive resin composition as the photo resist in exposing and development the resin layer layered on the surface of a CCL on which a circuit is formed. Thus, it is possible to form a hole or a window highly accurately in a predetermined position of the CCL. Furthermore, it is possible to use the resin layer as the cover lay film, by, if necessary, thermally curing the resin layer thereafter. To sum up, the photosensitive cover lay film can be used, not only as an insulating protective film, but also as the film-like shaped photo resist (photosensitive dry resist film). This makes it possible to improve workability and accuracy in positioning.
Especially, the use of a photosensitive cover lay film of dry film type gives the following advantages compared with the method in which the photosensitive resin composition is applied: the use of the photosensitive cover lay film of dry film type eliminates labor and time for applying and drying; and a large number of holes can be formed at once by developing the photosensitive cover lay film of dry film type. Thus, it is possible to manufacture the FPCs more efficiently.
However, the photosensitive resin composition and the photosensitive cover lay film are produced from (meth)acrylic type resin, generally. For example, photosensitive cover lay films whose main component is commercially available (meth)acrylic type resin, are known, which are disclosed in Japanese Publications of Unexamined Patent Applications, Tokukaihei No. 7-278492 (published on Oct. 24, 1995), Tokukaihei No. 7-253667 (published on Oct. 3, 1995), Tokukaihei No. 10-254132 (published on Sep. 25, 1998), Tokukaihei No. 10-115919 (published on May 6, 1998) and the like.
Moreover, apart from the (meth)acryl type resin, a photosensitive cover lay film whose main component is epoxy type resin is also known, for example.
However, those photosensitive cover lay films have such problems that the photosensitive cover lay films are, after curing, inadequate in soldering heat resistance, folding resistance (frangibility, flexing resistance), chemical resistance, electric insulating property, flame resistance, and the like. There has been a demand for improvement of those photosensitive cover lay films.
In view of this, arts in which a resin composition containing the photosensitive polyimide is used have been conventionally developed in order to solve the problems.
Specifically, for example, photosensitive polyimides into which a methacryloyl group is introduced via an ester bonding are disclosed in Japanese Publications of Patents, Tokukousho No. 55-030207 (Japanese Publication of Unexamined Patent application, Tokukaisho No. 49-115541, published on Nov. 5, 1974), and Tokukousho, No. 55-041422 (Japanese Publication of Unexamined Patent application, Tokukaisho No. 51-40922, published on Apr. 6, 1976).
Moreover, in Japanese Publications of Unexamined Patent Applications, Tokukaisho No. 54-145794 (published on Nov. 14, 1979), Tokukaisho No. 59-160140 (published on Sep. 10, 1984), Tokukaihei No. 03-170547 (published on Jul. 24, 1991), Tokukaihei 03-186847 (published on Aug. 14, 1991), Tokukaisho No. 61-118424 (published on Jun. 5, 1986), disclosed are photosensitive polyimides in which (a) an amine compound having a methacryloyl group, or (b) a diisocyanate compound is introduced at a position of a carboxyl group of a polyamic acid.
However, production of the photosensitive polyimides disclosed in those publications needs imidization of a polyamic acid after exposure to light and development. Therefore, there are (1) heat-causing problems: it is necessary to apply a temperature of 250° C. or higher on the FPCs. In general FPCs, application of a high temperature of 250° C. or higher changes crystalline structure of copper thereby deteriorating the circuit made of copper, and (2) problems associated with the imidization: some photosensitive polyimides require removal of an acryloyl group by heat, and the removal of the acryloly group causes signification reduction of film thickness.
Incidentally, besides being layered on the FPC, the photosensitive cover lay film is layered, for example, on a head section of a hard disc apparatus, and the like. The hard disc apparatus is used widely in personal computer and the like. To be used as such in those electronic parts and the like, a flame resistance is required for fear of inflammation due to long-time exposure to the electronic parts to a high temperature. Thus, the photosensitive cover lay film should have a high flame resistance.
As an art that also attempts to improve the flame resistance, Japanese publication of Unexamined Patent Application, Tokukai, 2001-335619, (published on Dec. 4, 2001) discloses a photosensitive cover lay film in which an epoxy-modified polyimide is used. The photosensitive cover lay film disclosed in the publication has sufficient heat resistance, chemical resistance, and folding resistance. However, the photosensitive cover lay film shows a poor fluidity during thermo compression bonding. Thus, it is necessary to press, with a high pressure, the photosensitive cover lay film in order to bond the photosensitive cover lay film with a CCL by pressing. Thus, the art is not practical.
Further, in using the photosensitive resin composition or the photosensitive cover lay film, how to expose them to the light and develop them is also an important factor. In general an alkali aqueous solution is used in light-exposing and development. It is an advantage that the photosensitive cover lay film whose main component is the (meth)acryl resin, can be surely developed because the (meth)acryl type resin has a carboxyl group, which is a hydrophilic group. However, as described above, the photosensitive cover lay film whose main component is the (meth)acryl resin has a problem that the film, after curing, becomes poor in heat resistance, chemical resistance, and folding resistance.
As described above, in case the photosensitive resin composition, or the photosensitive film produced therefrom is used in the electronic parts, it is not easy to attain sufficient properties both before and after curing. Thus, there is a limit in raw materials to use for the production of the photosensitive resin composition and the photosensitive film.
The present invention is contrived in view of the foregoing problems. An object of the present invention is to provide (a) a photosensitive resin composition that contains a polyimide, that has excellent properties, and that has a high practicability to be suitably used in electronic parts and the like, (b) a photosensitive cover lay, a film photosensitive film, and a laminate, which are produced from the photosensitive resin composition, the film photosensitive film being suitable for use as a photosensitive dry resist film.